Ridges

Ridges – Ridges are defined as “an isolated (or group of) elongated narrow elevation(s) of varying complexity having steep sides, often separating basin features” (IHO 2008). In this study “ridges” were confined to features greater than 1,000 m in relief (i.e. they overlay the abyssal mountains classification layer). Ridges partially overlap with the mid-ocean ridges, which were mapped as a separate feature, but in many areas mid-ocean ridges do not attain sufficient vertical relief (>1,000 m) to qualify as “ridges” in the present study.

In order to distinguish between seamounts and ridges, it is important to note the IHO (2008) definition of seamounts includes the specification that seamounts are “conical in form”. Thus features having a width/length ratio of < 0.5 are defined here as ridges; ridges are generally larger (mean size of 2,570 km2 versus 810 km2 for seamounts) and less steep-sided than seamounts. Escarpments (areas of seafloor dipping at an angle of over 5o) characterize 46.1% of ridge flanks compared with 63.4% of seamounts and guyots globally.

Overall, ridges are most common in the North and South Pacific Oceans, covering 3.50% and 3.00% of the two ocean regions, respectively. The Arctic Ocean and the Mediterranean and Black Sea have the fewest number of ridges and least amount of ridge area (< 1%). The largest single ridge feature mapped in this study is an un-named ridge near the Aleutian Islands that covers an area of 63,400 km2. It is interesting that, although geomorphic ridges are often associated with mid-ocean spreading (seafloor rifting), they cover the greatest areas in regions of plate convergence and subduction (eg. in the western Pacific and in Drake Passage).

Map of ridges – Note how ridges in the Atlantic and Indian Oceans coincide with the mid-ocean ridge systems whereas those in the Pacific and Southern Oceans are mostly unrelated to seafloor spreading ridges. The extensive ridge systems in the western Pacific are closely related to subduction zones and associated convergent margin plate tectonic processes.

Statistics of ridges (after Harris et al., 2014). The percentage areas refer to fraction of ocean regions that is ridge.

Ocean

Area km2

Ridges area%

Number of Ridges

Average Ridge Size km2

Number of Ridge summits <1000m

Area of Ridge summits <1000m km2

Arctic Ocean

118,050

0.909

61

1,940

78

17,707

Indian Ocean

1,747,480

2.45

608

2,870

288

145,389

Mediterranean & Black Sea

26,460

0.875

22

1,200

58

9,443

North Atlantic

990,440

2.21

447

2,220

386

63,366

North Pacific

2,873,990

3.50

1,089

2,640

806

280,912

South Atlantic

1,081,370

2.67

376

2,880

193

46,490

South Pacific

2,616,730

3.00

1,050

2,490

1108

375,385

Southern Ocean

316,200

1.55

176

1,800

91

18,408

All Oceans

9,770,720

2.70

3,796

2,570

3008

957,101

Ridge ecology

Ridges are important ecologically because rocky substrates are rare in the ocean, covering perhaps ~10% or less of the total area of ocean floor and because they provide such a different habitat from sedimentary seabeds, they tend to support unique faunal communities. Rocky geomorphic features are characterised by steep-sides, large vertical relief and because they may obstruct currents causing local acceleration of flow, commonly they experience stronger currents than sediment-covered substrates. Rocky seabeds may be associated with positive relief features (pinnacles, seamounts, guyots, ridges, escarpments, peaks, reefs, hills or mountains) or negative relief features (valleys, canyons, trenches, troughs, gullies, holes or deeps). Among this assortment of positive and negative relief features, detailed biological surveys have focussed on seamounts and submarine canyons, which can be generalised and extrapolated to the two categories to a certain extent.

Animals that live on top of coarse sediment or on hard surfaces (epifauna) are commonly filter-feeders. They are mostly sessile animals (attached to the seabed) that hold their feathery appendages into the passing current to trap food particles as it drifts by. Examples of sessile abyssal filter-feeding animals include the Coelenterata (hydras, anemones, corals and seapens) and Porifera (sponges). Deep, cold water coral communities are recognised as a particularly sensitive benthic ecosystem that is vulnerable to the effects of bottom trawling. Deep corals are often located on the flanks of seamounts but are also found on ridges and other high-relief features (eg. Mortensen et al., 2008).

Bathymetric image of Toogee Ridge, south of Tasmania, Australia (Exon et al., 1997) with arrows illustrating deflection of currents along the axis of the ridge thus providing ecological connectivity of habitats along the ridge crest.